Lightly beat the egg white in a shallow bowl and spread out the caster sugar on a baking sheet or tray. Dip the grapes and the holly or bay leaves into the egg white, or use a paintbrush. Shake off the excess, then coat in the sugar. Set aside to dry for at least 10 mins, ideally about 30 mins.
When you’re ready to decorate, make the frosting. Put a large bowl over a pan of simmering water, making sure the bottom of the bowl does not touch the water. Put the egg whites and icing sugar into the bowl and whisk for 5-7 mins until you have a thick, very glossy frosting. Use a spatula to clean around the edges of the bowl every so often as you whisk. Scoop the frosting onto the top of the cake and spread it around with a flat-edged knife, swirling as you go to create a snowy effect (see Knowhow, below).
Halve the clementines, figs and kumquats, then arrange on top of the frosted cake with the physalis, frosted grapes and leaves. Fix a ribbon around the base and leave the cake somewhere cool - but not the fridge - until your guests arrive.